1. Field of the Invention
The present invention relates to a cooling method of hydrotreating plant and a cooling unit therefor.
2. Description of Related Art
A general hydrotreating plant includes a desulfurization section having a reactor for generating hydrogen sulfide by hydrotreating sulfur and a hydrogen sulfide absorber for absorbing the hydrogen sulfide generated in the reactor, and a hydrogen sulfide absorbing section for circulating liquid for absorbing the hydrogen sulfide in the hydrogen sulfide absorber.
In the hydrotreating plant, the reactor should be rapidly cooled approximately from 400 to 40 degrees centigrade during plant shut down operation for the maintenance work such as catalyst replacement.
An air-cooling heat exchanger has been used as a cooling unit for cooling the hydrotreating plant. The air-cooling heat exchanger is provided on the downstream side of the reactor, and removes heat of a recycled gas circulated in a line of the air-cooling heat exchanger.
However, it takes five to seven days for the conventional method to cool the reactor of the hydrotreating plant from 400 to 40 degrees centigrade. Further, the term for cooling is influenced by an atmospheric temperature.
To solve the problem, some solutions have been developed such as a method in which the whole air-cooling heat exchanger is cooled by spraying water on the exchanger (improvement 1), a method in which a bypass is installed on the heat exchanger in the desulfurization section and a cooled recycled gas passing the bypass is directly introduced into the reactor (improvement 2), a method in which liquid nitrogen is supplied from the top of the reactor to lower the temperature of the recycled gas (improvement 3), a method in which liquid propane is supplied from the top of the reactor to lower the temperature of the recycled gas (improvement 4), a method in which a whole furnace is cooled by watering the top thereof (improvement 5), and a method in which circulating liquid is cooled by a special cooling unit provided on the hydrogen sulfide absorber and a circulating gas is cooled by the cooled circulating liquid passing through the hydrogen sulfide absorber, so that the whole reactor is cooled by the cooled circulating gas (improvement 6).
In the improvement 1, since the water spray damages a motor and a bearing of the air-cooling heat exchanger, a damage-preventing shield and a water-drop-preventing tub for the motor etc. are necessary. Further, a lot of water is necessary for adequately cooling the air-cooling heat exchanger.
In the improvement 2, high quality and thick pipes are required for the line, since the bypass is installed in the high-temperature and high-pressure desulfurization section, so that bypass installation cost is very expensive. Further, the bypass in itself doesn't bring enough cooling acceleration effect.
In the improvement 3, huge amount of liquid nitrogen is necessary for adequately cooling the plant, so that excessive running cost is needed.
In the improvement 4, it is necessary to purge out the evaporated propane gas to a flare by nitrogen gas after cooling.
In the improvement 5, since the inside of the furnace is heavily wetted with the water, it takes additional time for start up operation. Further, it is necessary to prevent the water from penetrating into a burner of the furnace. Furthermore, since the whole furnace is cooled by water, cooling efficiency is low and a lot of water is necessary for adequate cooling.
In the improvement 6, though more efficient cooling than the improvements 1 to 5 is possible, since special cooling unit is necessary, cost for preparing the unit and additional set-up work are necessary therefor.